In recent years, blue light emitting diodes formed of nitride semiconductor have been commercialized and furthermore blue laser diodes have also been in practical utilization.
In FIG. 15, a nitride semiconductor laser device fabricated by conventional art on a GaN substrate is shown in a schematic front view. This laser device includes a 4 μm thick n-type GaN contact layer 802, an n-type In0.08Ga0.92N crack prevention layer 803, a 1.2 μm thick n-type AlGaN clad layer 804 (having a superlattice structure including Al0.14Ga0.86N layers and GaN layers, and having a mixed-crystal composition of Al0.07Ga0.93N as averaged), a 0.075 μm thick n-type GaN guide layer 805, an active quantum well layer 806 (including three pairs of an In0.11Ga0.89N well layer and an In0.01Ga0.99N barrier layer), a p-type Al0.4Ga0.6N electron trapping layer 807, a 0.075 μm thick p-type GaN guide layer 808, a 0.5 μm thick AlGaN clad layer 809 (having a superlattice structure of Al0.1Ga0.9N layers and GaN layers, and having a mixed-crystal composition of Al0.05Ga0.95N as averaged), and a 15 nm thick p-type GaN contact layer 810, sequentially deposited on a GaN substrate 801.
FIG. 16 is a graph showing a radiation pattern in a direction vertical to the active layer (hereinafter referred to as a “vertical radiation pattern”) in a far field pattern (FFP) of optical radiation from the FIG. 15 laser device. More specifically, in this graph, a horizontal axis represents the deviation angle (degrees) from a direction parallel to the active layer toward a direction perpendicular to the active layer and a vertical axis represents the optical intensity (a.u.: arbitrary unit). When the substrate is made of a material having a refractive index larger than an effective refractive index of a waveguide as in the FIG. 15 laser device, then in a transverse mode vertical to the active layer (a vertical transverse mode), laser rays having reached the substrate are radiated through the substrate. As shown in FIG. 16, therefore, the FFP includes a noise peak at a direction deviated by about ten and several degrees from an emission direction of a fundamental mode (a direction parallel to the active layer) toward the substrate side (downward). A laser device causing such a noise peak involves a problem in application to optical disks etc. Further, such a noise peak corresponds to radiation loss of waveguide, involving problems of increase of threshold current in the laser device and decrease of differential quantum efficiency in lasing.
On the other hand, when thick (4 μm) n-type Al0.05Ga0.95N contact layer 802 is deposited between GaN substrate 801 and n-type AlGaN clad layer 804 as in the FIG. 15 laser device, the radiation (or leakage) mode to GaN substrate 801 tends to be suppressed. In that case, however, n-type AlGaN clad layer 804 must be formed to have a relatively large thickness of approximately 0.8 μm, and it becomes difficult to completely prevent cracks in its crystal. As a consequence, electric current leakage, increase of threshold current, and decrease of reliability are involved, causing decrease of the laser device yield rate.
Accordingly, a main object of the present invention is to suppress the noise peak in the vertical radiation pattern in such a semiconductor laser device including a substrate having a refractive index larger than an effective refractive index of a waveguide as in the case that a semiconductor laser structure is fabricated on a GaN substrate. Another object of the present invention is to prevent decrease of the nitride semiconductor laser device yield rate attributed to cracks in the n-type clad layer having a relatively large Al composition ratio, and to suppress the noise peak in the vertical radiation pattern.